Composite containing coated fibrous material

ABSTRACT

A composite is produced by depositing a slurry of infiltration-promoting material and organic binding material on a layer of boron nitride coated fibrous material forming a tape therewith on drying, firing the tape to burn out organic binding material and infiltrating the resulting porous body with a solution of boron and silicon.

This application is related to Ser. No. 056,516, filed June 1, 1987 forBorom et al., and Ser. No. 100,806, filed Sept. 24, 1987; Ser. No.216,488, filed July 8, 988; Ser. No. 216,471, filed July 8, 1988; Ser.No. 135,858, filed June 9, 1988; Ser. No. 132,753, filed June 9, 1988;Ser. No. 290,552, filed Dec. 27, 1988; all for Singh et al. Thereference applications are directed to the production of fibercontaining ceramic composites. They are assigned to the assignee hereofand are incorporated herein by reference.

U.S. Pat. Nos. 4,120,731; 4,141,948; 4,148,894; 4,220,455; 4,238,433;4,240,835, 4,242,106; 4,247,304, 4,353,953, 4,626,516, and 4,737,328;assigned to the assignee hereof and incorporated herein by reference,disclose silicon infiltration of materials which include carbon,molybdenum, carbon-coated diamond and/or cubic boron nitride, and blendsof carbon with silicon carbide, boron nitride, silicon nitride, aluminumoxide, magnesium oxide, and zirconium oxide.

This invention relates to the production of a composite containing boronnitride-coated fibrous material in a matrix containing a phase of asolution of boron and elemental silicon.

Fiber reinforcement of brittle ceramic materials offers significantopportunities for toughening of the brittle matrix. Many efforts havebeen extended to produce fiber reinforced, high temperature materials.Structures of carbon fiber reinforced carbon matrices have been used inhigh temperature structural applications but they have the disadvantageof poor to no oxidation resistance (i.e. they burn). High strengthcarbon fibers were infiltrated with molten silicon with the hope thatthe silicon matrix would protect the carbon filaments. However, thecarbon filaments converted instead into relatively weak, irregularcolumns of SiC crystals resulting in composites with low toughness andrelatively modest strength.

As an alternative approach, attempts have been made to incorporate SiCtype fibrous material in a silicon matrix by the process of siliconinfiltration. There are a number of problems when silicon carbidefibrous material is infiltrated with silicon. Even though SiC haslimited solubility in molten silicon, this solubility leads to transportand recrystallization of SiC thereby causing the SiC fibers to losestrength. Also, silicon carbide forms a strong bond with silicon whichresults in brittle fracture of the composite.

The present invention utilizes a solution of boron and elemental siliconto infiltrate a porous body comprised of boron nitride coated fibrousmaterial and infiltration promoting material to produce a composite inwhich the fibrous material has not been affected, or has not beensignificantly deleteriously affected, by processing conditions.

In one embodiment, since boron nitride is not wettable by silicon, acoating of a silicon-wettable material is deposited on the boron nitridecoating.

In another embodiment, boron nitride coated filaments of a sufficientlylarge diameter are used which do not require a silicon-wettable coating.Generally, these filaments or boron nitride coated filaments have adiameter of at least about 50 microns and a minimum length of at leastabout 10 times their diameter.

In one embodiment of the present process, a slurry ofinfiltration-promoting material and organic binder is cast onto a layerof the coated fibrous material to form a tape therewith, the tape isfired to burn out binder and then infiltrated with a molten solution ofboron and silicon.

In another embodiment of the present process, a plurality of the tapesare formed into a layered structure, then laminated, fired to burn outbinder and then infiltrated with a molten solution of boron and silicon.

The present invention permits control of the structure of the finalcomposite. It enables production of any desired spacing and alignmentwithin a layer and between layers of coated fibrous material in thematrix.

Those skilled in the art will gain a further and better understanding ofthe present invention from the detailed description set forth below,considered in conjunction with the accompanying figures which form apart of the specification, in which:

FIG. 1 shows a graph illustrating the load-deflection behavior of oneembodiment of the present composite comprised of spaced boron nitridecoated filaments in a matrix, and another graph illustrating theload-deflection behavior of a monolithic body; and

FIG. 2 is an optical micrograph of a polished cross-section of thecomposite whose load-deflection behavior is shown in FIG. 1.

Briefly stated, one embodiment of the present process for producing acomposite containing at least about 10% by volume of boron nitridecoated fibrous phase and having a porosity of less than about 20% byvolume comprises:

(a) forming a slurry of infiltration-promoting material and organicbinding material in a liquid medium;

(b) depositing a coating of boron nitride on fibrous material leaving nosignificant portion thereof exposed;

(c) depositing a silicon-wettable coating on said boron nitride-coatedfibrous material leaving no significant portion of said boron nitrideexposed;

(d) providing the resulting coated fibrous material substantially as alayer;

(e) casting said slurry onto said coated fibrous material in an amountsufficient to form a tape therewith;

(f) evaporating said liquid medium forming a tape;

(g) firing said tape to remove said organic binding material producing aporous body;

(h) providing an infiltrant comprised of boron and silicon containingelemental boron in solution in an amount of at least about 0.1% byweight of elemental silicon;

(i) contacting said porous body with infiltrant-associated infiltratingmeans whereby said infiltrant is infiltrated into said porous body;

(j) heating the resulting assembly in a partial vacuum to a temperatureat which said infiltrant is molten and infiltrating said molteninfiltrant into said porous body to produce an infiltrated product; and

(k) cooling said product producing said composite.

In a preferred embodiment, a plurality of the tapes are assembled toform a layered structure, the layered structure is laminated, and thelaminated structure is fired to remove the organic binding material toproduce a porous body which is then infiltrated with the molteninfiltrant.

As used herein "fibrous material" includes fibers, filaments, continuousfilaments, strands, bundles, whiskers, cloth, felt and any combinationthereof. The fibrous material can be amorphous, crystalline or a mixturethereof. The crystalline fibrous material can be single crystal orpolycrystalline.

In one embodiment, the fibrous material is selected from the groupconsisting of aluminum oxide, mullite, a carbon-containing material,elemental carbon, a SiC-containing material, silicon carbide, a siliconnitride-containing material, silicon nitride and mixtures thereof.

The carbon-containing material preferably contains carbon in an amountof at least about 1% by weight, frequently at least about 5% by weight,of the fibrous material.

As used herein, "elemental carbon" includes all forms of elementalcarbon including graphite.

Reference herein to a fibrous material of silicon carbide, includes,among others, presently available materials wherein silicon carbideenvelops a core or substrate, and which generally are produced bychemical vapor deposition of silicon carbide on a core or substrate suchas, for example, elemental carbon or tungsten.

The SiC-containing fibrous material generally contains at least about50% by weight of silicon and at least about 25% by weight of carbon,based on the weight of the fibrous material. Examples of SiC-containingmaterials are silicon carbide, Si--C--O, Si--C--O--N, Si--C--O--Metaland Si--C--O--N--Metal, where the Metal component can vary butfrequently is Ti or Zr and wherein O, N and Metal are present generallyin an amount of at least about 1% by weight of the fibrous material.

The silicon nitride-containing fibrous material generally contains atleast about 50% by weight of silicon and at least about 25% by weight ofnitrogen based on the weight of said fibrous material and is selectedfrom the group consisting of Si--N--O, Si--C--O--N, Si--N--O-Metal, andSi--C--O--N--Metal, wherein said O, C, and Metal are each present in anamount of at least about 1% by weight of said fibrous material.

There are processes known in the art which use organic precursors toproduce SiC- and silicon nitride-containing fibrous materials which mayintroduce a wide variety of elements into the fibrous material.

The fibrous material is stable at the temperature of the presentprocess. Preferably, the fibrous material has in air at ambient or roomtemperature, i.e. from about 20° C. to about 30° C., a minimum tensilestrength of about 100,000 psi and an minimum tensile modulus of about 25million psi.

In carrying out the present process, boron nitride is coated on thefibrous material to produce a coating therein which leaves nosignificant portion, and preferably none, of the fibrous materialexposed. The boron nitride coating should be continuous, and preferablyit is of uniform or at least significantly uniform thickness.

The boron nitride coating can be deposited on the fibrous material by anumber of known techniques under conditions which have no significantdeleterious effect on the fibrous material. Generally, the boron nitridecoating can be deposited by chemical vapor deposition by reactions suchas:

    B.sub.3 N.sub.3 H.sub.6 (g)→3BN(s)+3H.sub.2 (g)     (1)

    B.sub.3 N.sub.3 H.sub.3 Cl.sub.3 (g)→3BN(s)+3HCl(g) (2)

    BCl.sub.3 (g)+NH.sub.3 (g)→BN(s)+3HCl(g)            (3)

Generally, the chemical vapor deposition of boron nitride is carried outat temperatures ranging from about 900° to 1800° C. in a partial vacuum,with the particular processing conditions being known in the art ordeterminable empirically.

The boron nitride coating should be at least sufficiently thick to becontinuous. Generally, the coating is sufficiently thin so that thethermal expansion coefficient of the boron nitride coated fibrousmaterial is the same as, or not significantly different from, that ofthe uncoated fibrous material. Generally, the thickness of the coatingranges from about 0.3 microns to about 5 microns, and typically it isabout 0.5 microns. The particular thickness of the coating isdeterminable empirically, i.e. it should be sufficient to preventreaction, or prevent significant reaction, between the fibrous materialand the infiltrating infiltrant, i.e., its elemental silicon component,under the particular processing conditions used. The boron nitridecoating should leave none or no significant portion of the fibrousmaterial exposed. In the present invention, the boron nitride coatingbars contact, or bars significant contact, between the fibrous materialand the infiltrating infiltrant. During the infiltration process, theboron nitride coating may or may not react with or dissolve in themolten infiltrant depending on the amount of elemental boron in solutionin elemental silicon. When a saturated solution of boron and silicon isused as an infiltrant, the boron nitride coating will not react with ordissolve in the molten infiltrant. However, when an unsaturated solutionof boron and silicon is used as infiltrant, the boron nitride coatingmay or may not react with or dissolve in the molten infiltrant and thisis determinable empirically depending largely on time, temperature andconcentration of boron in solution. For example, for a given unsaturatedsolution, the boron nitride coating will survive better at lowertemperatures and/or shorter times. Generally, infiltration timeincreases with the size of the porous body. Larger sized porous bodies,therefore, may require thicker boron nitride coatings when theinfiltrant is an unsaturated solution. However, for a given infiltrationtime and temperature, as the concentration of boron in solution isincreased, the tendency of the boron nitride coating to react with ordissolve in the molten infiltrant usually decreases.

A number of techniques can be used to determine if the boron nitridecoating survived. For example, if the composite exhibits fiber pull-outon fracture, then the boron nitride coating has survived. Also, scanningelectron microscopy of a cross-section of the present composite candetect a boron nitride coating on the fibrous material.

In one embodiment, a coating of a silicon-wettable material is depositedon the boron nitride coating substantially covering it. Preferably, thesilicon-wettable coating leaves no significant portion or none of theboron nitride coating exposed. Also, preferably, the silicon-wettablecoating is of uniform or at least significantly uniform thickness.Generally, the thickness of the silicon-wettable coating ranges fromabout 500 Angstroms to about 3 microns, and typically it is about 0.5microns. Its particular thickness is determined empirically and dependslargely on its rate of consumption, if any, and the particular compositedesired.

The silicon-wettable coating is a solid in the present process. Itsparticular composition depends largely on the particular compositedesired and is determined empirically. It should be sufficiently wettedby the infiltrating infiltrant to enable production of the presentcomposite. Preferably, the infiltrant has a contact or wetting angleagainst the silicon-wettable material of less than 90 degrees to allowthe infiltration to occur by capillarity. The silicon-wettable coatingshould have no significant deleterious effect herein.

Representative of useful silicon-wettable materials is elemental carbon,metal carbide, a metal which reacts with silicon to form a silicide, ametal nitride such as silicon nitride, and a metal silicide. Elementalcarbon is preferred and usually, it is deposited on the boronnitride-coated material in the form of pyrolytic carbon. Generally, themetal carbide is a carbide of silicon, tantalum, titanium or tungsten.Generally, the metal silicide is a silicide of chromium, molybdenum,tantalum, titanium, tungsten and zirconium.

The metal which reacts with silicon to form a silicide thereof as wellas the silicide must have melting points higher than the melting pointof silicon and preferably higher than about 1450° C. Representative ofsuch metals is chromium, molybdenum, tantalum, titanium, tungsten andzirconium.

Known techniques can be used to deposit the coating of silicon-wettablematerial which generally is deposited by chemical vapor deposition usinglow pressure techniques.

The metal carbide or metal silicide coating can be directly depositedfrom the vapor thereof. Alternatively, the metal carbide coating can beformed in situ by initially depositing carbon followed by deposition ofmetal thereon under conditions which form the metal carbide. If desired,metal silicide coating can be produced by initially depositing the metalfollowed by deposition of silicon under conditions which form the metalsilicide.

In carrying out the present process, the coated fibrous material, i.e.,the fibrous material coated with boron nitride alone or with boronnitride and silicon-wettable material, can be provided substantially asa layer by a number of conventional techniques. For example, a layer ofthe coated fibrous material can be deposited on a flat surface. Theamount of coated fibrous material is determined empirically and shouldbe sufficient to produce the final composite containing at least about10% by volume of boron nitride coated fibrous material. The layer ofcoated fibrous material can be continuous or discontinuous and itcontains sufficient spacing to permit production of the presentcomposite. There is sufficient spacing between the coated fibers,filaments, strands, bundles, or whiskers to permit penetration thereofby the infiltration-promoting material sufficiently to produce thepresent composite. The extent of spacing in the layer of coated fibrousmaterial is determined empirically and depends largely on the size ofthe infiltration-promoting material and the particular compositedesired.

The infiltration-promoting material is also a silicon-wettable materialwhich is sufficiently wetted by the infiltrating infiltrant to enablethe production of the present composite. Preferably, the infiltratinginfiltrant has a contact or wetting angle against theinfiltration-promoting material of less than 90 degrees to allow theinfiltration to occur by capillarity. The infiltration-promotingmaterial should have no significant deleterious effect in the presentprocess or on the resulting composite. The particular composition of theinfiltration-promoting material is determined empirically and dependslargely on the particular composite desired, i.e. the particularproperties desired in the composite.

Representative of useful infiltration-promoting materials is elementalcarbon, a metal carbide, a metal which reacts with silicon to form asilicide, a metal nitride, a metal silicide and mixtures thereof.

Elemental carbon is a preferred infiltration-promoting material.Preferably, elemental carbon comprises from about 5% to about 100%, orfrom about 10% to about 90%, by volume of the infiltration-promotingmaterial.

Representative of a useful infiltration promoting metal carbide is boroncarbide, molybdenum carbide, niobium carbide, silicon carbide, titaniumcarbide and mixtures thereof. Silicon carbide is preferred.

In a preferred embodiment, the infiltration-promoting material iscomprised of a mixture of elemental carbon and silicon carbide, eachbeing present in an amount of at least about 10% by volume of theinfiltration-promoting material.

Representative of the infiltration-promoting metal nitrides is aluminumnitride, niobium nitride, silicon nitride, titanium nitride, zirconiumnitride and mixtures thereof.

Representative of a metal which reacts with silicon to form a silicideis chromium, molybdenum, tantalum, titanium, tungsten, zirconium andmixtures thereof. Likewise, representative of infiltration-promotingmetal silicides is chromium silicide, molybdenum silicide, tantalumsilicide, titanium silicide, tungsten silicide, zirconium silicide andmixtures thereof. The metal which reacts with silicon to form a silicidethereof as well as the silicide must have melting points higher than themelting point of the infiltrant and preferably higher than about 1450°C.

Generally, the infiltration-promoting material is a solid in the presentprocess. However, in one embodiment, elemental silicon forms a part ofthe infiltration-promoting material and ranges up to about 30%,preferably from about 1% to about 10%, by volume of theinfiltration-promoting material. Elemental silicon, as a part of theinfiltration-promoting material, is molten when the infiltrant isinfiltrated into the porous structure and promotes it infiltration.

The infiltration-promoting material is in a form useful for forming aslurry therewith and for forming the desired tape with the layer ofcoated fibrous material. A sufficient amount of theinfiltration-promoting material is of a size, and preferably it iscomprised of a size, which can penetrate the spaces in the layer ofcoated fibrous material sufficiently to produce the present composite.

The infiltration-promoting material can be in the form of a powder, afibrous material and mixtures thereof. When the infiltration-promotingmaterial is in the form of particles, generally the particles have aspecific surface area ranging from about 0.2 to about 10 meters² pergram, and frequently, ranging from about 2 to about 4 meters² per gram.When the infiltration-promoting material is in the form of a fibrousmaterial, it generally is in the form of whiskers, chopped fibers andmixtures thereof. Generally, the whiskers are less than about 50 micronsin length and less than about 10 microns in diameter. Preferably, thewhiskers are crystalline and are comprised of silicon carbide, siliconnitride and mixtures thereof. Generally, the chopped fibers are lessthan about 50 microns in length and less than about 10 microns in width.Preferably, the chopped fibers are comprised of graphite. The amount andtype of infiltration-promoting material depends largely on theparticular composite desired and is determined empirically.

The organic binding material used in the present process bonds theinfiltration-promoting material together to form a tape of desiredthickness and solids content. By solids content, it is meant herein thecontent of infiltration-promoting material and coated fibrous material.The organic binding material, i.e. that component of the tape other thanits solids content, thermally decomposes at an elevated temperatureranging to below about 800° C., generally from about 50° C. to belowabout 800° C., and preferably from about 100° C. to about 500° C., togaseous product of decomposition which vaporizes away leaving nosignificant deleterious residue.

The organic binding material is a thermoplastic material with acomposition which can vary widely and which is well known in the art orcan be determined empirically. Besides an organic polymeric binder itcan include an organic plasticizer therefor to impart flexibility. Theamount of plasticizer can vary widely depending largely on theparticular binder used and the flexibility desired, but typically, itranges up to about 50% by weight of the total organic content.Preferably the organic binding material is soluble in a volatilesolvent.

Representative of useful organic binders are polyvinyl acetates,polyamides, polyvinyl acrylates, polymethacrylates, polyvinyl alcohols,polyvinyl butyrals, and polystyrenes. The useful molecular weight of thebinder is known in the art or can be determined empirically. Ordinarily,the organic binder has an average molecular weight at least sufficientto make it retain its shape at room temperature and generally such anaverage molecular weight ranges from about 20,000 to about 200,000,frequently from about 30,000 to about 100,000.

Representative of useful plasticizers are dioctyl phthalate, dibutylphthalate, diisodecyl glutarate, polyethylene glycol and glyceroltrioleate.

In carrying out the present process, the infiltration-promoting materialand organic binding material are admixed with a liquid medium to form asuspension or slurry which preferably is uniform or at leastsubstantially uniform. A number of conventional techniques can be usedto form the slurry. Generally, the components are milled in an organicsolvent in which the organic material is soluble or at least partiallysoluble to produce a castable suspension or slurry, i.e. a slurrysuitable for depositing on the layer of coated fibrous material to forma tape therewith. Examples of suitable solvents are methyl ethyl ketone,toluene and alcohol. The particular composition of the slurry isdetermined empirically. Generally, at least about 10% by volume of theslurry is comprised of infiltration-promoting material.

The tape can be cast by a number of conventional techniques. Preferably,the layer of coated fibrous material is deposited on a carrier fromwhich the resulting tape can be easily released such as Teflon®. Theslurry can be deposited on the layer of coated fibrous material to forma tape therewith of desired thickness and solids content which isdetermined empirically. Frequently, the slurry is cast on the layer ofcoated fibrous material by doctor blading. The cast tape is dried toevaporate the solvent therefrom to produce the present tape which isthen removed from the carrier.

The particular amount of organic binding material used in forming theslurry is determined empirically and depends largely on the amount anddistribution of solids desired in the resulting tape. Generally, theorganic binding material ranges from about 25% by volume to about 80% byvolume, frequently from about 25% to about 50% by volume, of the solidscontent of the tape.

The present tape or sheet can be as long and as wide as desired, andgenerally it is of uniform or substantially uniform thickness. Itsthickness depends largely on the volume fraction of coated fibrousmaterial which must be accommodated and the particular composite desiredand is determinable empirically. The tape should be at leastsufficiently thick to contain an amount of infiltration-promotingmaterial and coated fibrous material to produce the desired composite.Generally, with increasing volume fractions of coated fibrous material,correspondingly smaller amounts of infiltration-promoting material wouldbe required. Generally, the tape has a thickness ranging from about 25microns (0.001 inch) to about 1300 microns (0.052 inch), frequentlyranging from about 125 microns (0.005 inch) to about 1000 microns (0.040inch), and more frequently ranging from about 250 microns (0.01 inch) toabout 500 microns (0.02 inch).

Generally, the tape is flexible or may be made flexible by theapplication of heat, and can be bent to form a desired shape.

In one embodiment of the present process, the coated fibrous material iscomprised of boron nitride coated filaments preferably with a diameterranging from about 50 microns to about 250 microns, frequently fromabout 70 microns to about 200 microns, or from about 100 microns toabout 150 microns. This boron nitride coated filament is continuous andcan be as long as desired. It has a minimum length of at least about 10times its diameter, and generally, it is longer than about 1000 microns,or it is longer than about 2000 microns. The minimum diameter of thecoated filament depends largely on the minimum spacing required betweenthe coated filaments through which the infiltration-promoting materialmust penetrate and is determined empirically. For a given volumefraction of coated filaments, as the diameter of the coated filamentdecreases, the total amount of space between coated filaments decreasesmaking it more difficult for the infiltration-promoting material topenetrate the space.

Preferably, in one embodiment, a preform comprised of a layer of aplurality of continuous boron nitride coated filaments which are spacedfrom each other and which are parallel, or at least substantiallyparallel, to each other is used. The minimum space between the coatedfilaments is at least sufficient to enable the infiltration-promotingmaterial to penetrate therebetween, and generally, it is at least about50 microns, and frequently at least about 100 microns. Generally, thespacing between coated filaments in a single layer is substantiallyequivalent, or if desired, it can vary. Filament loading in thecomposite can be varied by changing the spacing between the coatedfilaments and/or tape thickness. In a preferred embodiment, thefilaments are comprised of silicon carbide or elemental carbon and theslurry contains whiskers of silicon carbide. The present inventionenables the production of a final composite with a high volume fractionof uniaxially aligned boron nitride coated continuous filaments.

The preform of coated filaments can be produced by a number ofconventional techniques. For example, the coated filaments can beuniaxially aligned and spaced by placing them in a suitable deviceprovided with grooves and the desired spacing. The layer of coatedfilaments can be lifted off the device with adhesive tape placed acrossboth ends of the filaments. The slurry can then be deposited on thelayer of coated filaments to produce a tape therewith.

In a preferred embodiment of carrying out the present process, aplurality of the tapes are assembled into a layered structure. Thenumber of tapes used can vary widely depending largely on the particularcomposite desired. Preferably, the tapes in the layered structure are atleast substantially coextensive with each other, i.e. substantially asandwich-type structure.

In one embodiment, before assembly of the layered structure, a solutionof the present organic binder in organic solvent is deposited, generallysprayed, on the faces of the tapes to be contacted with each other,dried to evaporate the solvent and leave a sticky film of organic binderto enhance adhesion. The concentration of organic binder in solution canvary widely and generally ranges from about 1% by weight to about 10% byweight of the solution. The solution is sprayed on the face of the tapefor a period of time, determinable empirically, so that on evaporationof the solvent sufficient sticky binder remains to significantly enhanceadhesion or facilitate bonding of the tapes. Preferably, drying iscarried out in air at ambient temperature in less than a minute, andtypically, in a few seconds. The deposited binder can be a continuous ora discontinuous coating, and typically, 0.2 milligrams of sticky binderper square centimeter of surface is adequate.

The layered structure is then laminated under a pressure and temperaturedetermined empirically depending largely on the particular compositionof the organic binding material to form a laminated structure.Lamination can be carried out in a conventional manner. Laminatingtemperature should be below the temperature at which there isdecomposition, or significant decomposition, of organic binding materialand generally, an elevated temperature below 150° C. is useful and thereis no significant advantage in using higher temperatures. Typically, thelamination temperature ranges from about 35° C. to about 95° C. and thepressure ranges from about 500 psi to about 3000 psi. Generally,lamination time ranges from about 1/2 to about 5 minutes. Also,generally, lamination is carried out in air.

If desired, the laminated structure can be cut to desired dimensions bysuitable means such as a diamond saw. Frequently, the laminatedstructure is flexible and can be bent to form a desired shape dependinglargely on fiber diameter and thickness of the laminated structure andmay require some heat to impart flexibility thereto.

The laminated structure, or in another embodiment individual tape, isheated to thermally decompose the organic binding material thereinproducing a porous body comprised of the coated fibrous material andinfiltration-promoting material. The rate of heating depends largely onthe thickness of the sample and on furnace characteristics. At a firingtemperature ranging up to about 500° C., a slower heating rate isdesirable because of the larger amounts of gas generated at thesetemperatures by the decomposition of the organic binding material.Typically, the heating rate for a sample of less than about 6millimeters (6000 microns) in thickness can range from about 15° C. perhour to about 30° C. per hour. At a temperature of less than about 800°C., thermal decomposition is completed leaving no significantdeleterious residue.

Thermal decomposition can be carried out in any atmosphere, preferablyat about or below atmospheric pressure, which has no significantdeleterious effect on the sample such as, for example, argon.Preferably, thermal decomposition is carried out in a partial vacuum toaid in removal of gases.

The resulting porous body is comprised of coated fibrous material andinfiltration-promoting material. The porous body has a structure and anopen porosity which permit production of the present composite.Generally, the open porosity is distributed in or through the body. Byopen porosity it is meant herein pores, voids, or channels which areopen to the surface of the body thereby making the interior surfacesaccessible to the ambient atmosphere or the infiltrant. The particularamount of open porosity is determined empirically and depends largely onthe particular final composite desired. Generally, the porous body hasan open porosity ranging from about 25% by volume to about 80% by volumeof the body. Frequently, the porous body has an open porosity rangingfrom about 35% by volume to about 75% by volume, or from about 40% byvolume to about 50% by volume, of the body. Preferably, the openporosity enables the molten silicon to infiltrate by capillarity.

Preferably, the open porosity is comprised of small pores, preferablyranging from about 0.1 micron and about 50 microns, and preferably atleast significantly or substantially uniformly distributed in or throughthe porous body thereby enabling the production of a composite whereinthe matrix phase is at least significantly or substantially uniformlydistributed in or through the composite. Also, this would produce acomposite wherein the matrix phase has a thickness between the boronnitride coated fibers or fibrous material ranging from about 0.1 micronto about 50 microns.

Generally, the porous body has no closed porosity. By closed porosity itis meant herein closed pores or voids, i.e. pores not open to thesurface of the body and therefore not in contact with the ambientatmosphere.

Void or pore content, i.e. both open and closed porosity, can bedetermined by standard physical and metallographic techniques.

The present infiltrant is comprised of boron and silicon wherein boronranges generally from about 0.1% by weight to about 10% by weight,frequently from about 1% by weight to about 10% by weight, andpreferably from about 1% by weight to about 3% by weight, of silicon.Boron ranging from about 0.1% by weight to about 1.6% by weight ofsilicon is in solution in silicon, and at about 1.6% by weight it formsa saturated solution. In excess of about 1.6% by weight of silicon,boron forms a compound therewith which precipitates as a finelydispersed solid. Amounts of boron in excess of about 10% by weight ofsilicon provide no advantage. When the infiltrant is molten, theprecipitate usually is SiB₆. When the infiltrant is solid, theprecipitate can be SiB₃, SiB₆, or a mixture thereof. The compounds ofboron and silicon have no significant effect on the present process,i.e. they are substantially inert herein. Preferably, the infiltrant isa saturated solution.

The infiltrant can be formed in a known manner. For example, a solidparticulate mixture of boron and silicon can be heated in an atmospherenon-oxidizing with respect to silicon to a temperature at which siliconis molten and boron will dissolve therein.

In carrying out the present process, the porous body is contacted withinfiltrant-associated infiltrating means whereby the infiltrant isinfiltrated into the body. The infiltrating means allows the molteninfiltrant to be infiltrated into the body. For example, an assembly isformed comprised of the porous body in contact with means that are incontact with the solid infiltrant and which permit infiltration of theinfiltrant, when molten, into the porous body. In one infiltrationtechnique, the porous body is placed on a woven cloth of elementalcarbon, a piece of infiltrant is also placed on the cloth, and theresulting assembly is heated to infiltration temperature. Atinfiltration temperature, the molten infiltrant migrates along the clothand wicks into the porous body. After infiltration, the wicking carboncloth may be removed from the composite by diamond grinding.

In another technique, the infiltration procedure can be carried out asset forth in U.S. Pat. No. 4,626,516 which discloses an assembly thatincludes a mold with infiltration holes and a reservoir holdingelemental silicon. A preform is placed within the mold and carbon wicksare provided in the infiltrating holes. The wicks are in contact withthe preform and also with the silicon and at infiltration temperaturethe molten silicon migrates along the wicks into the preform.

U.S. Pat. No. 4,737,328 discloses another infiltration technique whichcomprises contacting a preform with a powder mixture composed of siliconand hexagonal boron nitride, heating the resulting structure to atemperature at which the silicon is fluid and infiltrating the fluidsilicon into the preform. After infiltration, the hexagonal boronnitride powder is brushed off the composite.

The present assembly is heated to infiltration temperature in anonoxidizing partial vacuum wherein the residual gases have nosignificantly deleterious effect on said assembly and the presentinfiltration is carried out in such nonoxidizing partial vacuum.Preferably, such nonoxidizing partial vacuum is provided before heatingis initiated. The partial vacuum should be at least sufficient to avoidthe entrapment of pockets of gas which would lead to excessive porosity,i.e. it should be sufficient to produce the present composite.Generally, such a partial vacuum ranges from about 0.01 torr to about 2torr, and usually from about 0.01 torr to about 1 torr to insure removalof entrapped gas in the body being infiltrated.

Ordinarily and as a practical matter, the furnace used is a carbonfurnace, i.e. a furnace fabricated from elemental carbon. Such a furnaceacts as an oxygen getter for the atmosphere within the furnace reactingwith oxygen to produce CO or CO₂ and thereby provides a nonoxidizingatmosphere, i.e. the residual gases have no significantly deleteriouseffect on the infiltrating infiltrant. The present infiltration cannotbe carried out in air because the liquid silicon would oxidize and forma dense silica coating before any significant infusion by the infiltrantoccurred. In such instance where a carbon furnace is not used, it ispreferable to have an oxygen getter present in the furnace chamber, suchas elemental carbon, in order to insure the maintenance of anonoxidizing atmosphere. Alternatively, other nonoxidizing atmosphereswhich have no significant deleterious effect on the structure within thefurnace can be used at partial vacuums of about 10⁻² torr to 2 torr.

The present infiltration is carried out at a temperature at which theinfiltrant is molten and which has no significant deleterious effect onthe porous body being infiltrated. The present infiltration temperatureranges from a temperature at which the infiltrant is molten to atemperature at which there is no significant vaporization of theinfiltrant. Molten silicon has a low viscosity. The melting point of thesilicon, and therefore the melting point of the present infiltrant, canvary depending largely on the particular impurities which may bepresent. Generally, the present infiltration temperature ranges fromgreater than about 1400° C. to about 1550° C., and preferably from bout1450° C. to about 1500° C. The rate of penetration of the infiltrantinto the porous body depends on the setting of the body by theinfiltrant melt and the fluidity of the melt. With increase intemperature, the ability of the molten infiltrant to wet the porous bodyimproves.

In the present process, sufficient infiltrant is infiltrated into theporous body to produce the present composite. The molten infiltrant ismobile and highly reactive. It has an affinity for silicon-wettablematerials wetting them or wetting and reacting with them. It has astrong affinity for elemental carbon, wetting it and reacting with it toform silicon carbide. The molten infiltrant also has an affinity for anymetal with which it reacts to form the silicide thereof. In addition,sufficient infiltrant is infiltrated into the porous body to fill poresor voids which may remain to produce the present composite.

The period of time required for infiltration by the infiltrant isdeterminable empirically and depends largely on the size of the porousbody and extent of infiltration required. Generally, it is completed inless than about 20 minutes, and often in less than about 10 minutes.

The resulting infiltrated body is cooled in an atmosphere and at a ratewhich has no significant deleterious effect on it. Preferably, it isfurnace cooled in the nonoxidizing partial vacuum to about roomtemperature, and the resulting composite is recovered.

In the present process, there is no loss, or no significant loss, of thecoated fibrous material and infiltration-promoting material.

The present composite is comprised of boron nitride-coated fibrousmaterial and a matrix phase. The matrix phase is distributed in orthrough the boron nitride-coated fibrous material and generally it issubstantially completely space filling. Usually, the matrix phase iscontinuous and interconnecting. Generally, the matrix in the finalcomposite has a thermal expansion coefficient ranging from lower thanthat of the boron nitride coated fibrous material to less than about 15%higher than that of the boron nitride coated fibrous material. Theproduction of such a composite can be determined empirically. Morepreferably, for optimum mechanical properties of the composite, thematrix has a thermal expansion coefficient ranging from less than toabout the same as that of the boron nitride coated fibrous material.

Generally, the matrix is in direct contact with more than 70% of thesurface area of the boron nitride coated fibrous material. Frequently,the matrix coats or envelops each coated fiber, filament, strand, bundleor whisker of the boron nitride coated fibrous material sufficiently tobe in direct contact with more than 80%, preferably more than 90%, morepreferably more than 99%, of the surface area of the boron nitridecoated fibrous material in the composite. Most preferably, the boronnitride-coated fibrous material is substantially or totally enveloped bythe matrix phase.

Generally, the boron nitride coated fibrous material ranges from about10% to about 70%, frequently from about 20% to about 60% or from about30% to about 50%, by volume of the composite.

The boron nitride coating on the fibrous material in the composite isdetectable by scanning electron microscopy and generally ranges inthickness from about 0.5 microns to about 1.5 microns. The particularamount of boron nitride in the composite provided by the boron nitridecoating depends largely on the amount of coated fibrous materialpresent, the thickness of the boron nitride coating and the diameter ofthe fiber, filament, or whisker. Therefore, the volume fraction of boronnitride provided by the coating is the balance of the volume fraction ofall other components of the composite. Frequently, however, the boronnitride coating on the fibrous material in the composite generallyranges from less than about 1% by volume to about 20% by volume, or fromabout 1% by volume to about 10% by volume, or from about 1% by volume toabout 5% by volume, of the total volume of boron nitride coated fibrousmaterial. The boron nitride coating can be amorphous crystalline or acombination thereof.

Generally, the boron nitride coating optimizes interfacial shear stressbetween the fibrous material and matrix resulting in a composite with atoughness significantly higher than that of a composite wherein thefibrous material is not coated with boron nitride.

Generally, there is no significant amount of reaction product formed,and preferably no reaction product detectable by scanning electronmicroscopy, directly between the matrix and the fibrous material coatedwith boron nitride.

The matrix comprises at least about 30% by volume of the composite. Thecomposition of the matrix can vary and depends largely on thecomposition of the infiltration-promoting material and anysilicon-wettable coating. Preferably, at least about 70% or at least90%, or at least 95%, or more than 99%, by volume of the matrix iscrystalline. Most preferably,. the matrix is crystalline. In oneembodiment, the matrix has an average grain size of less than about 100microns, or less than about 50 microns, or less than about 20 microns,and most preferably less than about 10 microns. In another embodiment,part of the matrix is comprised of crystalline whiskers ranging up toabout 50% by volume of the matrix. Preferably, the whiskers arecomprised of silicon carbide, silicon nitride, and mixtures thereof.

In one embodiment, the matrix is a ceramic selected from the groupconsisting of metal carbide, metal nitride, metal silicide and mixturesthereof.

Preferably the metal carbide in the matrix is selected from the groupconsisting of boron carbide, molybdenum carbide, niobium carbide,silicon carbide, titanium carbide and mixtures thereof. Silicon carbideis most preferred. Silicon carbide phase formed in situ may containboron.

Preferably, the metal nitride in the matrix is selected from the groupconsisting of aluminum nitride, niobium nitride, silicon nitride,titanium nitride, zirconium nitride and mixtures thereof.

Preferably, the metal silicide in the matrix is selected from the groupconsisting of chromium silicide, molybdenum silicide, tantalum silicide,titanium silicide, tungsten silicide, zirconium silicide, and mixturesthereof. Metal silicide phase formed in situ may contain boron.

Generally, the matrix contains a phase comprised of a solution of boronand elemental silicon in at least a detectable amount, i.e. an amountdetectable by microprobe analysis or Auger electron spectroscopy. In oneembodiment, the phase comprised of a solution of boron and siliconranges from a detectable amount to about 50% by volume, or from about 1%by volume to about 5% by volume, or to about 2% by volume, of thematrix. In the solution phase, boron ranges from about 0.1% by weight toabout 1.6% by weight of silicon. Generally, this phase is distributed inthe composite, and preferably, it is distributed through the compositesignificantly uniformly.

The present composite may contain a phase of a compound of boron andsilicon usually selected from the group consisting of SiB₃, SiB₆, and amixture thereof which generally is distributed through the composite.The compound of boron and silicon usually ranges from an amountdetectable by microprobe analysis up to about 30% by volume, or up toabout 5% by volume, or up to about 1% by volume, of the matrix.

The matrix may contain a phase of a metal which forms a silicide butwhich had not reacted with the infiltrant. In such instance, it would beencapsulated by a metal silicide phase and/or a boron-containing metalsilicide phase. Generally, such metal may range from an amountdetectable by microprobe analysis, or from about 0.5% by volume to about10% by volume, of the matrix.

The matrix of the present composite may contain a phase of elementalcarbon which had not completely reacted with the infiltrating silicon.In such instance, the carbon would be totally encapsulated by a phase ofsilicon carbide and/or boron-containing silicon carbide formed in situ.Generally, this elemental carbon may range from an amount detectable bymicroprobe analysis, or from about 0.5% by volume, to about 10% byvolume, of the composite.

In one embodiment, the present composite is comprised of matrixcontaining a layer of boron nitride coated fibrous material.

In another embodiment, the present composite is comprised of matrixcontaining a plurality of layers of boron nitride coated fibrousmaterial. Preferably, the coated fibrous layers are substantiallyparallel to each other and separated from each other by matrix.Preferably, the matrix is distributed in each layer of coated fibrousmaterial generally significantly or substantially uniformly.

In another embodiment, the composite is comprised of matrix containing aplurality of layers of boron nitride coated filaments which preferablyare substantially parallel to each other and are separated from eachother by matrix. In each layer, preferably more than 95% by volume, ormore than 99% by volume, of the coated filaments, and preferably all orsubstantially all of the coated filaments, are spaced from each otherand parallel or at least substantially parallel, to each other.Preferably, more than 95% by volume, or more than 99% by volume, orsubstantially all, of the coated filaments in each layer are aligned, orsubstantially aligned, in a single plane. Any misalignment of the coatedfilaments should not significantly degrade the mechanical properties ofthe composite. Also, preferably more than 95%, or more than 99%, orsubstantially all, of the surface area of the coated filaments is indirect contract with the matrix. Also, crystalline whiskers may bepresent up to about 50% by volume of the matrix.

The boron nitride coating optimizes interfacial shear stress between thefilaments and matrix resulting in a composite with a toughnesssignificantly or substantially higher than that of a composite whereinthe filaments are uncoated. Specifically, if the matrix and filamentswere in direct contact, even a slight reaction therebetween wouldincrease interfacial bonding thereby requiring a higher stress to pullout the filaments making the composite less tough. If the interfacialbonding were too high, then the composite would fail in a brittlemanner. In contrast, the present boron nitride coating provides aninterfacial shear stress which is significantly lower than that producedwith uncoated filaments thereby allowing the coated filaments to pullout more easily and gives the composite more toughness. The coatedfilaments prevent brittle fracture of the composite at room temperature.By brittle fracture of a composite it is meant herein that the entirecomposite cracks apart at the plane of fracture. In contrast to abrittle fracture, this embodiment of the composite exhibits filamentpull-out on fracture at room temperature. Specifically, as thiscomposite cracks open, generally at least about 10% by volume,frequently at least about 30% or 50% by volume, of the coated filaments,and preferably all of the coated filaments, pull out and do not break atthe plane of fracture at room temperature.

One particular advantage of this invention is that the present compositecan be produced directly in a wide range of sizes. For example, it canbe as long or as thick as desired.

Also, since the tape, sheet or laminated structure used to produce thepresent composite frequently is flexible, or may be made flexible byapplication of heat thereto, the present composite can be comprised of avariety of shapes such as tape, sheet, bent tape, bent sheet or anycombination thereof. Therefore, the present composite can be of a formwhich is simple, complex, hollow or any combination thereof.

The present composite has a porosity of less than about 20%, preferablyless than about 10%, or less than about 5%, more preferably less thanabout 1%, by volume of the composite. Most preferably, the composite isvoid- or pore-free, or has no significant porosity, or has no porositydetectable by scanning electron microscopy. Preferably, any voids orpores in the composite are less than about 70 microns, more preferablyless than about 50 microns or less than about 10 microns, and generallythey are distributed in the composite.

The present composite has a wide range of applications depending largelyon its particular composition. For example, it is useful as a wearresistant part, acoustical part or high-temperature structuralcomponent.

The invention is further illustrated by the following examples where,unless otherwise stated, the procedure was as follows:

The infiltrant was produced by forming a mixture of boron and siliconpowders wherein boron was present in an amount of about 3% by weight ofsilicon. The mixture was heated in a vacuum non-oxidizing with respectto silicon to about 1450° C. and boron dissolved in the molten siliconforming a saturated solution as well as a finely divided precipitate ofa compound of boron and silicon. The melt was then cooled to roomtemperature in the same vacuum. The resulting solid was then broken intosmall chunks.

Commercially available continuous filaments of silicon carbide producedby a chemical vapor deposition process and sold under the trademark AVCOSCS-6 were used. These filaments had a 35 micron carbon core on whichsilicon carbide was deposited to an overall diameter of about 145microns. The outside surface of the filaments consisted of two layers ofpyrolytic carbon and carbon-silicon, with overall thickness of about 3microns. In air at room temperature these filaments have a tensilestrength of about 500 thousand psi and a tensile modulus of about 60million psi. These filaments have an average thermal expansioncoefficient of less than about 5.0×10⁻⁶ in/in-° C.

The filaments were cut to a length of about 2 inches and were coatedwith boron nitride by the following low pressure chemical vapordeposition process utilizing the reaction B₃ N₃ H₃ Cl₃ →+3BN+3HCl.Specifically, the filaments were placed on a molybdenum screen which wasthen positioned at about the midpoint of the hot zone of apyrex/quartz/pyrex furnace tube A 1.00 gram sample of commercialtrichloroborazine (B₃ N₃ H₃ Cl₃) was transferred in an argon filledglove box to a pyrex end-section which contained a thermocouple vacuumgauge, a cold trap and a vacuum stopcock. The closed pyrex end-sectionwas then taken out of the glove box and attached to an end of thefurnace tube and to a vacuum system. The end-section containing thetrichloroborazine was then cooled using liquid nitrogen and the furnacetube was opened to the vacuum system via the stopcock of the pyrexend-section. After the system reached a pressure lower than 0.020 torr,the furnace was heated to about 1050° C. When the pressure had againdropped below 0.020 torr and the furnace temperature had stabilized, theend-section containing the trichloroborazine was warmed by an oil bathmaintained at 60° C., whereupon the solid began to vaporize, depositingBN and liberating gaseous HCl in the hot zone of the furnace tube andproducing an increase in pressure. The pressure was observed to reach ashigh as about 200 torr before stabilizing at about 50 torr. After twohours, the pressure was found to have decreased to about 0.020 torr,whereupon the furnace was shut down and the system allowed to cool toroom temperature before opening the tube and removing the sample.Identification of the chemically vapor deposited layer as BN wasaccomplished by means of electrical resistance measurement and aquantitative ESCA analysis of a film deposited in substantially the samemanner on a SiC disk surface. This film was amorphous to x-rays in theas-deposited condition and appeared fully dense and smooth at highmagnification in the SEM. Scanning electron microscopy observation ofthe ends of coated and broken filaments revealed that the coating wascontinuous and smooth and about 1.5 microns thick on the filament andleft no significant portion of the filament exposed.

The boron nitride coated filaments were uniaxially aligned by placingthem in a device for aligning filaments and maintaining the requiredspacing between them. This device was made from a copper foil laminatedon a printed circuit board which was etched by the photolithographictechnique in such a way as to produce parallel grooves about 0.06 inchdiameter, 0.0004 inch deep, and 0.0008 inch apart (center-to-center).The coated filaments were placed on this device and a simple scoop ofthe filaments using a straight edge led to filling of each of thegrooves with a filament. This resulted in a single layer of uniformlyspaced coated filaments which was lifted off the bard by puttingadhesive tapes across each end portion of the filament layer. Theadhesive tapes were sufficient to maintain the alignment and spacingbetween the coated filaments in the layer. Several such pre-formedlayers of coated filaments were produced in which the coated filamentswere substantially parallel and spaced about 100 microns from eachother.

The silicon carbide powder had an average surface area of about 5 m² /g.

The crushed carbon felt was comprised of chopped fibers sufficientlysmall to form a slurry.

By ambient temperature herein it is meant room temperature, i.e. fromabout 20° C. to about 30° C.

The organic binding material was comprised of commercially availableorganic binder comprised of polyvinylbutyral (average molecular weightof about 32,000) and commercially available liquid plasticizer comprisedof polyunsaturated hydroxylated low-molecular weight organic polymers.

Standard techniques were used to characterize the hot pressed compositefor density, microstructure and mechanical properties.

EXAMPLE 1

72 grams of a crushed carbon felt and 48 grams of silicon carbide powderwere dry mixed with 1.5 grams of organic binder. To this, 70 grams of amixture consisting of toluene and methyl isobutyl ketone in the ratio3:1 was added. This mixture was mixed for 5 minutes in a paint shaker.Then, 4.5 grams of the organic binder and 4.5 grams of liquidplasticizer was added to the mixture. To this, 0.1 gram of acommercially available organic silicone oil (which can be considered apart of the organic binding material) was added. This mixture was ballmilled for about 30 minutes at room temperature to form a slurry. Theslurry was then deaired in a vacuum. Each pre-formed layer of boronnitride coated filaments was deposited on a Kodacel sheet, the slurrywas deposited on the filaments using a doctor blade, the cast tape wasthen dried in air at room temperature and atmospheric pressure to removethe solvent, and the resulting tape was stripped from the Kodacel sheet.

The tape was about 6 inches wide and had a substantially uniformthickness of about 0.012 inch.

The tape was cut to the length and width of the aligned layer offilaments. A number of such tapes were produced.

A layered sandwich-type structure was formed comprised of four layers oftape. Before assembly, to enhance adherence, the faces of the tapeswhich were to be contacted with each other were sprayed with an organicsolution of binder, and dried for a few seconds in air at roomtemperature leaving a coating of sticky organic binder. Specifically, asolution comprised of 3 weight % of commercially availablepolyvinylbutyral (average molecular weight of about 32,000), 39 weight %toluene, 9.5 weight % acetone, 39 weight % xylene and 9.5 weight %ethanol was used. The solution was sprayed on the faces of the tapes fora sufficient time so that upon evaporation of the solvent there remainedabout 0.2 milligrams of sticky organic binder per square centimeter ofsurface.

The resulting layered structure was laminated in air in a laminatingpress at about 93° C. under a pressure of about 1000 psi for about oneminute.

The laminated structure was sliced perpendicular to the filament axisinto a bar-shaped sample (1.50 inch ×0.3 inch ×0.10 inch) using adiamond saw. Examination of a cross-section showed uniform spacingbetween the filaments as well as between the layers of filaments.

The sample was placed in a vacuum oven for removing the organic bindingmaterial wherein the vacuum was typically about 20 millitorr. Theburn-out cycle was comprised of heating the furnace at a rate of 30° C.per hour to 500° C., a five-hour hold at 500° C. and a cool-down to roomtemperature at a rate of 200° C. per hour. This led to complete removalof the organic binding material from the laminated structure whichresulted in a porous body comprised of elemental carbon and boronnitride coated filaments. From other work, it was known that the porousbody had an open porosity of about 50% by volume.

The porous body and solid pieces of infiltrant were placed on a wovencarbon fabric which was contained in a BN-sprayed graphite tray. Thistray was then placed in a carbon resistance heated bell jar furnace andslowly heated at a rate of about 10° C. per minute to about 400° C. in avacuum of about 0.05 torr. Subsequent to this, the porous body wasrapidly heated to 1420° C. at which point the infiltrant was molten andreacted with the carbon cloth and got wicked into the porous body. Aconsiderable amount of heat which was detected by a thermocouple placedon top of the porous body was generated due to the exothermic reactionof infiltrant with carbon fibers in the porous body. The sample was heldfor 5 minutes under these conditions and then slowly cooled to below themelting point of the infiltrant after which the furnace power was turnedoff and the infiltrated sample was cooled to room temperature in thevacuum of the bell jar.

The resulting composite was broken at room temperature in athree-point-bend configuration to determine fracture strength andload-elongation characteristics. It exhibited filament pullout, i.e.more than 10% by volume of the filaments pulled out and did not break atthe plane of fracture.

FIG. 1 shows a load deflection curve for this composite. It can be seenthat this composite showed toughened ceramic-like behavior. Theload-deflection curve shows that after the onset of matrix cracking, theload carrying capability of the composite was maintained for a whilereaching an ultimate strength of 61,916 psi (427 MPa) beyond which thecomposite showed substantial but not complete failure. This compositewould be useful as a structural or wear-resistant part.

A polished cross-section (perpendicular to filament axis) of thecomposite, which is illustrated in FIG. 2, was examined. It showeduniform spacing between the filaments. It also showed that each layer offilaments was maintained in a substantially single plane. In addition,it showed a fully dense matrix surrounding each individual boron nitridecoated filament and in direct contact therewith. No porosity wasdetected by microscopy.

It was estimated that, based on the volume of the composite, thiscomposite was comprised of about 65% by volume of silicon carbide phasesome of which may be boron-containing silicon carbide phase, about 15%by volume of a phase comprised of a solution of elemental boron andelemental silicon which contains boron in an amount of about 1.6% byweight of silicon, a minor amount of a compound of boron and silicon,and about 20% by volume of boron nitride coated filaments.

From other work, it was known that the average thermal expansioncoefficient of the matrix was lower than that of the boron nitridecoated filaments.

EXAMPLE 2

This example was carried out in substantially the same manner as Example1, except that no filaments were used.

For comparison, four tapes comprised of silicon carbide powder, crushedcarbon felt, and organic binding material were produced, formed into alayered sandwich-type structure, laminated, heated to remove organicbinding material, and melt infiltrated in substantially the same manneras the sample of Example 1 to produce a monolithic body of substantiallythe same size which was broken in substantially the same manner. Itsload deflection curve is also shown in FIG. 1. It fractured in a brittlemanner at 29,120 psi (200 MPa).

What is claimed and sought to be protected by Letters Patent of theUnited States is as follows:
 1. A process for producing a compositecontaining at least about 10% by volume of boron nitride coated fibrousmaterial and having a porosity of less than about 20% by volume whichcomprises:(a) forming a slurry of infiltration-promoting material andorganic binding material in a liquid medium; (b) depositing a coating ofboron nitride on fibrous material leaving no significant portion thereofexposed; (c) depositing a silicon-wettable coating on said boronnitride-coated fibrous material leaving no significant portion of saidboron nitride exposed; (d) providing the resulting coated fibrousmaterial substantially as a layer; (e) casting said slurry onto saidcoated fibrous material in an amount sufficient to form a tapetherewith; (f) evaporating said liquid medium forming a tape; (g) firingsaid tape to remove said organic binding material producing a porousbody; (h) providing an infiltrant comprised of boron and siliconcontaining elemental boron in solution in silicon in an amount of atleast about 0.1% by weight of elemental silicon; (i) contacting saidporous body with infiltrant associated infiltrating means whereby saidinfiltrant is infiltrated into said porous body; (j) heating theresulting assembly in a partial vacuum to a temperature at which saidinfiltrant is molten and infiltrating said molten infiltrant into saidporous body to produce an infiltrated product; and (k) cooling saidproduct producing said composite.
 2. The process according to claim 1,wherein said silicon-wettable material and said infiltration-promotingmaterial are comprised of elemental carbon.
 3. The process according toclaim 1, wherein said infiltration-promoting material is comprised of amixture of at least 10% by volume of elemental carbon and at least 10%by volume of silicon carbide.
 4. The process according to claim 1,wherein said fibrous material is selected from the group consisting ofaluminum oxide, mullite, a carbon-containing material, elemental carbon,a SiC-containing material, silicon carbide, a silicon nitride-containingmaterial, silicon nitride and mixtures thereof.
 5. The process accordingto claim 1, wherein said fibrous material is comprised of elementalcarbon.
 6. The process according to claim 1, wherein said fibrousmaterial is a silicon carbide-containing material.
 7. The processaccording to claim 1, wherein said composite has a porosity of less thanabout 10% by volume.
 8. A process for producing a composite containingat least about 10% by volume of boron nitride coated fibrous materialand having a porosity of less than about 20% by volume whichcomprises:(a) forming a slurry of infiltration-promoting material andorganic binding material in a liquid medium; (b) depositing a coating ofboron nitride on fibrous material leaving no significant portion thereofexposed; (c) depositing a silicon-wettable coating on said boronnitride-coated fibrous material leaving no significant portion of saidboron nitride exposed; (d) providing the resulting coated fibrousmaterial substantially as a layer; (e) casting said slurry onto saidcoated fibrous material in an amount sufficient to form a tapetherewith; (f) evaporating said liquid medium forming a tape; (g)assembling a plurality of said tapes to produce a layered structure; (h)laminating the layered structure to form a laminated structure; (i)firing said laminated structure to remove said organic binding materialproducing a porous body; (j) providing an infiltrant comprised of boronand silicon containing elemental boron in solution in silicon in anamount of at least about 0.1% by weight of elemental silicon; (k)contacting said porous body with infiltrant associated infiltratingmeans whereby said infiltrant is infiltrated into said porous body; (l)heating the resulting assembly in a partial vacuum to a temperature atwhich said infiltrant is molten and infiltrating said molten infiltrantinto said porous body to produce an infiltrated product; and (m) coolingsaid product producing said composite.
 9. The process according to claim8, wherein said silicon-wettable material and saidinfiltration-promoting material are comprised of elemental carbon. 10.The process according to claim 8, wherein said infiltration-promotingmaterial is comprised of a mixture of at least about 10% by volume ofelemental carbon and at least about 10% by volume of silicon carbide.11. The process according to claim 8, wherein said fibrous material isselected from the group consisting of aluminum oxide, mullite, acarbon-containing material, elemental carbon, a SiC-containing material,silicon carbide, a silicon nitride-containing material, silicon nitride,and mixtures thereof.
 12. The process according to claim 8, wherein saidfibrous material is comprised of elemental carbon.
 13. The processaccording to claim 8, wherein said fibrous material is a siliconcarbide-containing material.
 14. The process according to claim 8,wherein said composite has a porosity of less than about 10% by volume.15. A process for producing a composite containing at least about 10% byvolume of boron nitride coated filaments and having a porosity of lessthan about 20% by volume which comprises:(a) forming a slurry ofinfiltration-promoting material and organic binding material in a liquidmedium; (b) depositing a coating of boron nitride on a filamentproducing a boron nitride coated filament having a diameter of at leastabout 50 microns leaving no significant portion of said filamentexposed; (c) providing a plurality of the resulting coated filamentssubstantially as a layer wherein said coated filaments are spaced fromeach other; (d) casting said slurry onto said coated filaments in anamount sufficient to form a tape therewith; (e) evaporating said liquidmedium forming a tape; (f) firing said tape to remove said organicbinding material producing a porous body; (g) providing an infiltrantcomprised of boron and silicon containing elemental boron in solution insilicon in an amount of at least about 0.1% by weight of elementalsilicon; (h) contacting said porous body with infiltrant-associatedinfiltrating means whereby said infiltrant is infiltrated into saidporous body; (i) heating the resulting assembly in a partial vacuum to atemperature at which said infiltrant is molten and infiltrating saidmolten infiltrant into said porous body to produce an infiltratedproduct; and (j) cooling said product producing said composite.
 16. Theprocess according to claim 15, wherein said infiltration-promotingmaterial is comprised of elemental carbon.
 17. The process according toclaim 15, wherein said infiltration-promoting material is comprised of amixture of at least about 10% by volume of elemental carbon and at leastabout 10% by volume of silicon carbide.
 18. The process according toclaim 15, wherein said fibrous material is selected from the groupconsisting of aluminum oxide, mullite, a carbon-containing material,elemental carbon, a SiC-containing material, silicon carbide, a siliconnitride-containing material, silicon nitride, and mixtures thereof. 19.The process according to claim 15, wherein said fibrous material iscomprised of elemental carbon.
 20. The process according to claim 15,wherein said fibrous material is a silicon carbide-containing material.21. The process according to claim 15, wherein said composite has aporosity of less than about 10% by volume.
 22. A process for producing acomposite containing at least about 10% by volume of boron nitridecoated filaments and having a porosity of less than about 20% by volumewhich comprises:(a) forming a slurry of infiltration-promoting materialand organic binding material in a liquid medium; (b) depositing acoating of boron nitride on a filament producing a boron nitride coatedfilament having a diameter of at least about 50 microns leaving nosignificant portion of said filament exposed; (c) providing a pluralityof said boron nitride coated filaments substantially as a layer whereinsaid filaments are spaced from each other; (d) casting said slurry ontosaid coated filaments in an amount sufficient to form a tape therewith;(e) evaporating said liquid medium forming a tape; (f) assembling aplurality of said tapes to produce a layered structure; (g) laminatingthe layered structure to form a laminated structure; (h) firing saidlaminated structure to remove said organic binding material producing aporous body; (i) providing an infiltrant comprised of boron and siliconcontaining elemental boron in solution in silicon in an amount of atleast about 0.1% by weight of elemental silicon; (j) contacting saidporous body with infiltrant-associated infiltrating means whereby saidinfiltrant is infiltrated into said porous body; (k) heating theresulting assembly in a partial vacuum to a temperature at which saidinfiltrant is molten and infiltrating said molten infiltrant into saidporous body to produce an infiltrated product; and (l) cooling saidproduct producing said composite.
 23. The process according to claim 22,wherein said infiltration-promoting material is comprised of elementalcarbon.
 24. The process according to claim 22, wherein saidinfiltration-promoting material is comprised of a mixture of at leastabout 10% by volume of elemental carbon and at least about 10% by volumeof silicon carbide.
 25. The process according to claim 22, wherein saidfibrous material is selected from the group consisting of aluminumoxide, mullite, a carbon-containing material, elemental carbon, aSiC-containing material, silicon carbide, a silicon nitride-containingmaterial, silicon nitride, and mixtures thereof.
 26. The processaccording to claim 22, wherein said fibrous material is comprised ofelemental carbon.
 27. The process according to claim 22, wherein saidfibrous material is a silicon carbide containing material.
 28. Theprocess according to claim 22, wherein said composite has a porosity ofless than about 10% by volume.